Two common types of microwave transmission lines are coaxial transmission lines and microstrip transmission lines. A special type of coaxial line is known as rectangular coaxial line. In this type of line, an outer conductor shield having a rectangular cross-sectional configuration is used instead of an outer conductor shield with a circular cross-section which is used for conventional coaxial line. The inner conductor for rectangular coaxial line can also have either a rectangular cross-section or a circular cross-section. Rectangular coaxial lines are described, for example, in Microwaves, April, 1968, pp. 52-56, "Why Not Use Rectangular Coax?", W. S. Metcalf.
One type of rectangular coaxial transmission line is known as "modified square-ax"; it is a rectangular transmission line with a square outer conductor and a round inner conductor separated by a dielectric material.
It is desirable for some applications to use more than one type of transmission lines to interconnect individual circuits or devices for signal propagation. There is therefore a need to provide a transition between circuits or devices which include different types of transmission lines, and particularly between modified square-ax and microstrip transmission lines. One problem is the significant mismatch encountered at the interface between the two transmission lines due to the physical discontinuity. Many RF applications require transitions between different transmission line configurations/media with a minimum reflection of energy.